Diagnostic method for root-cause analysis of FET performance variation

Abstract

A diagnostic method of and computer system for root-cause analysis of performance variations of FETs in integrated circuits and a method and computer system for monitoring a field effect transistor manufacturing process. The diagnostic method includes measuring source currents in the linear and saturated regions of two FETs, calculating ratios of the source currents in the linear and saturated regions for the and two FETs and comparing the ratios of the two FETs to determine a probable root cause for a performance variation between the two FETs. One of the FETs has a known good performance.

1. A computer program product, comprising:
 non-transitory computer readable storage medium containing computer instructions stored therein for causing a computer processor to perform a method for diagnosing the presence of and determining the root cause of a performance variation of a second field effect transistor from a first field effect transistor, said method comprising the steps of:
 (a) measuring a linear threshold voltage of said first field effect transistor, said linear threshold voltage being a first gate voltage level at which a source current begins to flow; 
 (b) measuring a saturated threshold voltage of said first field effect transistor, said saturated threshold voltage being a second gate voltage level marking a boundary between a linear operating region of said first field effect transistor where said source current is substantially proportional to applied drain voltage and a saturated operating region of said first field effect transistor where said source current is substantially constant with respect to applied drain voltage; 
 (c) based on said linear threshold voltage, measuring a first linear source current of said first field effect transistor at a third gate voltage at which source current is substantially proportional to applied drain voltage; 
 (d) based on said saturated threshold voltage, measuring a first saturated source current of said first field effect transistor at a fourth gate voltage at which source current is substantially constant with respect to applied drain voltage; 
 (e) based on said linear threshold voltage, measuring a second linear source current of said second field effect transistor at said third gate voltage; 
 (f) based on said saturated threshold voltage, measuring a second saturated source current of said second field effect transistor at said fourth gate voltage; 
 (g) comparing a first ratio of said first linear source current to said first saturated source current to a second ratio of said second linear source current to said second saturated source current; and 
 (h) displaying or storing results of said comparing. 
 
 
     
     
       2. The computer program product of  claim 1 , wherein
 method step (c) includes setting said third gate voltage equal to said linear threshold voltage plus a constant voltage; 
 method step (e) includes setting said third gate voltage equal to said linear threshold voltage plus said constant voltage; 
 method step (d) includes setting said fourth gate voltage equal to said saturated threshold voltage plus said constant voltage; and 
 method step (f) includes setting said fourth gate voltage is equal to said saturated threshold voltage plus said constant voltage. 
 
     
     
       3. The computer program product of  claim 2 , wherein
 method step (c) includes choosing said constant voltage so said linear threshold voltage plus said constant voltage is about equal to a normal operating voltage level of said first field effect transistor. 
 
     
     
       4. The computer program product of  claim 2 , wherein
 method step (a) includes applying to a drain of said first field effect transistor, a first drain voltage not equal to zero volts and equal to about 10% or less of said normal operating voltage level of said first field effect transistor; 
 method step (b) includes applying to said drain of said first field effect transistor, a second drain voltage about equal to said normal operating voltage level of said first field effect transistor; 
 method step (c) includes applying zero volts to a source of said first field effect transistor and applying said first drain voltage to said drain of said first field effect transistor; 
 method step (d) includes applying zero volts to said source of said first field effect transistor and applying said second drain voltage to said drain of said first field effect transistor; 
 method step (e) includes applying zero volts to a source of said second field effect transistor and applying said first drain voltage to a drain of said second field effect transistor; and 
 method step (f) includes applying zero volts to the source of said first field effect transistor and applying said second drain voltage to said drain of said second field effect transistor. 
 
     
     
       5. The computer program product of  claim 1 , further including the method step of:
 based on said comparing, selecting a probable root cause a variance in performance of between said first field effect transistor and said second field effect transistor from a set of root causes and displaying said probable root cause on a display unit of said computer screen. 
 
     
     
       6. A computer program product, comprising:
 non-transitory computer readable storage medium containing computer instructions stored therein for causing a computer processor to perform a method for monitoring a field effect transistor manufacturing process, said method comprising the steps of:
 (a) measuring a linear threshold voltage of a base field effect transistor, said linear threshold voltage being a first gate voltage level at which a source current begins to flow; 
 (b) measuring a saturated threshold voltage of said base field effect transistor, said saturated threshold voltage being a second gate voltage level marking a boundary between a linear operating region of said first field effect transistor where said source current is substantially proportional to applied drain voltage and a saturated operating region of said first field effect transistor where said source current is substantially constant with respect to applied drain voltage; 
 (c) based on said linear threshold voltage, measuring a first linear source current of said base field effect transistor at a third gate voltage at which drain current is substantially proportional to applied gate voltage; 
 (d) based on said saturated threshold voltage, measuring a first saturated source current of said base field effect transistor at a fourth gate voltage at which drain current is substantially constant with respect to applied gate voltage; 
 (e) selecting an additional field effect transistor; 
 (f) based on said linear threshold voltage, measuring a second linear source current of said additional field effect transistor at said third gate voltage, said second field effect transistor manufactured after said first field effect transistor; 
 (g) based on said saturated threshold voltage, measuring a second saturated source current of said additional field effect transistor at said fourth gate voltage; 
 (h) comparing a first ratio of said first linear source current to said first saturated source current and comparing a second ratio of said second linear source current to said second saturated source current; and 
 (i) storing or displaying results of said comparing. 
 
 
     
     
       7. The computer program product of  claim 6 , wherein
 method step (c) includes setting said third gate voltage equal to said linear threshold voltage plus a constant voltage; 
 method step (f) includes setting said third gate voltage equal to said linear threshold voltage plus said constant voltage; 
 method step (d) includes setting said fourth gate voltage equal to said saturated threshold voltage plus said constant voltage; and 
 method step (g) includes setting said fourth gate voltage is equal to said saturated threshold voltage plus said constant voltage. 
 
     
     
       8. The computer program product of  claim 7 , wherein
 method step (c) includes choosing said constant voltage so said linear threshold voltage plus said constant voltage is about equal to a normal operating voltage level of said first field effect transistor. 
 
     
     
       9. The computer program product of  claim 7 , wherein
 method step (a) includes applying to a drain of said first field effect transistor, a first drain voltage not equal to zero volts and equal to about 10% or less of said normal operating voltage level of said first field effect transistor; 
 method step (b) includes, applying to said drain of said first field effect transistor, a second drain voltage about equal to said normal operating voltage level of said first field effect transistor; 
 method step (c) includes, applying zero volts to a source of said first field effect transistor and applying said first drain voltage to said drain of said first field effect transistor; 
 method step (d) includes, applying zero volts to said source of said first field effect transistor and applying said second drain voltage to said drain of said first field effect transistor; 
 method step (f) includes, applying zero volts to a source of said second field effect transistor and applying said first drain voltage to a drain of said second field effect transistor; and 
 method step (g) includes, applying zero volts to the source of said first field effect transistor and applying said second drain voltage to said drain of said second field effect transistor. 
 
     
     
       10. The computer program product of  claim 6 , further including the method step of:
 displaying results of said comparing on a display unit of said system.